Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to an implant for an improved endoprosthesis,
which is compatible with the structural requirements and conditions of the
skeleton and which results in a strong connection between the prosthesis and
the bone immediately after the surgical insertion operation, without the use
of bone cement, and using wear and corrosion-resistant materials which are
also toxically unobjectionable. Such a spherical cap prosthesis may be used,
either as part of a complete prosthesis - for which an acetabulum is also
necessary - or the spherical cap may be used as a partial prosthesis - in
which case, the external diameter of the ball is made larger and engages in
the natural acetabulum.
An artificial join~ replacement of sintered oxide ceramic for these
and similar fields of application has already been described in German
Offenlegungsschrift DT-OS 2 318 459. A joint head on a worked femur head,
or a joint roller, is there described which is in engagement with the artific-
ial ~oint socket and which also consists of sintered high melting point
metallic oxides. This structure, however, leads to a number of difficulties
in practice. Under actual conditions, the femur head can only poorly be
worked into a prismatic polygon. The resulting wide tolerances arising then
do not permit a close fit of the implant. Moreover, there is a danger of bone
fracture in the region of the neck of the femur as a result of the unspecific
loading, since considerable torques occur when the implant is screwed on with
the aid of the self-tapping thread.
It is furthermore apparent from a report by F. Betzel: "Kunststoffe
in der plastischen Chirurgie der Gelenke" (Plastics in plastic surgery of the
joints) in "Melsunger medizinisch-pharmazeutische Mitteilungen", Issue 100
(1963), pages 2478 - 86, that endoprostheses were introduced by the Judet
brothers with the first clinical applications in 1948. In particular, the
report describes types made of metal and plastic, and also plastic hip surgery
,
with Pyrex-Bakelite* caps by Smith-Peterson. Through pain caused by loading
or stress and by movement, the occurrence of infections, material damage and,
in some cases, also because of degenerative shrinkage of the head and neck
stump of the bone, it was necessary that
* both Pyrex and Bakelite are trade marks
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these plastic surgery caps be removed at an early stage. Such a cap
cannot be expected to remain on the bone stump for a lengthy period as the
cap fits only loosely.
In German Utility Model DT-GM 75 09 026, an attempt to eliminate
the foregoing disadvantages by uniting this floating mounting of the cap
closely and securely with the joint head with as little bone cement as
possible is described. The cap consists of metal in which surface unevenness
is provided on the inside with an opening to the outside.
Cap-shaped composite endoprostheses of metal and plastic are shown
in Swiss Patent Specification 560,538 and German Patent Specification DT-PS
876 739. Here the artificial joint head is spherical or mushroom-shaped
and the anchoring stem or shank mounted on the underside may simply be
straight or curved.
Although these implants permit careful treatment of the bone and also
do not particularly aggravate bleeding conditions during the surgery, they
have not yet been successful in practice because the metal and plastics
materials employed do not result in long-anticipated life of the implanted
endoprosthesis. Experience with ceramic implants has shown that design
improvements have by no means yet been exhausted. In fact,in use of these
artificial joint replacement parts, various disadvantages have become
manifest which can be traced back to design of the hip joint pros~hesis.
Thus, the problem which the inventors confronted was to provide a
one- or multi-part implant for an improved endoprosthesis which comp~ies
with the structural conditions of the skeleton in an improved manner which
also results in a strong connection between the prosthesis and the bone
immediately after the surgical operation without employing bone cement while
employing wear and corrosion-resistant materials which are also non-toxic.
An improvement towards simplified manufacture of the implant was also
required. And with the new implant, resection of the bone is minimal
resulting in decreased surgical operative intervention.
This problem is met in making the central opening of the spherical
cap of conical form and mounting over the conically worked femur head and
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femur neck as an exact fit. A thrust plate transfers the forces between
the femur head and the neck of the femur.
More particularly in accordance with the invention there is provided,
an implant for an endoprosthesis for mounting on the prepared neck of a femur
which comprises a spherical surfaced cap member, the cap including a frusto
conical central opening therein defining an inside surface for mounting over
the neck of the femur, and a thrust plate for engaging the inside surface
of said opening and received in the femur for transferring thrust forces
from said femur to said cap.
In the spherical cap member the central opening may have circular
or spiral grooves.
In a variation of the spherical cap, the internal cone of the central
opening may have axial recesses.
The inner surface of the spherical cap may also have axial pro-
jections.
The thrust plate may have a projection as a supplementary mechanical
retaining means. The thrust plate may advantageously be provided with a
threaded hole in the longitudinal direction for the purpose of effecting
supplementary fixing to the bone by means of a screw.
An oxide ceramic, preferably highly pure aluminium oxide, may be
employed for the spherical cap and also for the thrust plate.
A description of specific embodiments of the invention now follows
with reference to the accompanying drawings in which;
Figure 1 is a central longitudinal section through a femur with a
novel prosthesis mounted thereon;
Figures 2, 3 and 4, respectively, show sections along lines II-II,
III-III, and IV-IV, of Figure l;
Figures 5 and 6 show plan views of the embodiment of the two
respective thrust plates with alternatively arranged projections.
As can be seen in Pigure 1, a ceramic implant consists of a spherical
cap 1 with an optionally insertable thrust plate 4 which is secured in turn
to the prepared femur 10 by means of a draw screw 13. In preparing the
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1 damaged femur 10, the natural spherical shape of the femur head 2 is altered
to a cylindrical shape with a key-hole type saw capable oE curved cutting.
This working or dressing is effected in a plurality of stages. The process
is begun using a larger key-hole type saw and in so doing, it is determined
whether the cylindrical stub is located centrally with respect to the neck
of the femur, if necessary, any off-centre is rectified with a smaller key-
hole type saw. The cylindrical stub is then formed into a cone with a
special tool3 the cone ratio being between 1:10 and 1:6. This ratio is
defined as the difference between the dlameters of the cone at two perpendi-
cularly separated cross-sections divided by the perpendicular distance of
separation of such two sections. Next the height of the cone is determined
with a suitable gauge and any projecting edge is cut away. To increase
further the hold which the spherical cap 1 later to be attached, will hav~
on the conically worked femur head 2, a thrust plate 4 is inserted in the
bone lO. The free spaces for the thrust plate 4 and also for the draw screw
13 are made with a drill and chisel.
To secure the implant on the prepared femur, the thrust plate 4
is first placed in the free space and the cap 1 is pushed over it. The
cap 1 having a notching frusto conical surface is then forced on to the
conical bone stump together with the thrust plate 4. The implant is now
seated in closely conforming manner on the conical bone stump and, by means
of the thrust plate 4, can also be further secured by means of a surgical
screw 13 which can be introduced into the threaded hole 11 and tightened.
A washer 14 may be employed between the head 13' of screw 13 and the bone 10.
In Figures 2 and 3, the circular recesses 6 and the axial
recesses 7 for effecting the securing of the prosthesis to the bone can be
seen. Both forms of possible surface contouring in the inner part of the
spherical cap 1 permit durable anchoring of the implant to the bone by the
natural bone tissue growth and the adhesion of the tissue concerned being
promoted by these recesses and projections 8. The projections 8 serve at the
same time for the primary fixing of the implant and secure it against rotary
movement. Figure 4 shows the threaded hole 15 in which screw 13 is received.
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1 Figures 5 and 6 show alternative arrangements for projections 9
and 9' on two respective thrust plates 4 and 4'.
The upper part of each thrust plate 4 is of flattened tapered
form, whereas the lower part is more elliptical. Grooves 11 (see also Fig. 4)
assist in durable anchoring of the implant part by invasion or ingrowth of
bone substance, just as in the case of the recesses 6 and 7. The projections
9 and 9' are directed so that the back of the thrust plate 4 or 4' is applied
in closely conforming relation to the circular profile of the bone stump and
engages in a corresponding recess in the spherical cap 1. The effect of the
projections 9 or 9' is to prevent the cap 1 being pulled off the prepared
femur head 2. The projection 9' is rotated through 90 with respect to
- projection 9, but the function of the two is similar.
In order to obtain a friction and wear-resistant implant, the
surface of the spherical cap 1 is polished.
An advantage of the cap-shaped form of construction for the
ceramic implant is that biocompatible materials may be used and cement-free
implantations can be provided which is particularly advantageous for young
patients and results in a hip joint prosthesis which is both durable and
stable. By using the spherical ceramic cap, surgical operative measures can
be minimi~ed and firm seating of the implant can be ensured by working the
bone to fit accurately. Due to the conical shaping between the implant and
the bone, an improvement in the mechanical force flow is obtai~ed. In
practice, it is found that loading of the implant is possible immediately
after the operative procedures, 90 that the length of stay in hospital can be
considerably shortened. The closely conforming or mating shape of the implant,
as well as the working of the hip joint bone associated therewith, result in
an optimum pressure to the femur, enable bone tissue damage to be minimized
or eliminated. In comparison with all stem or shank protheses, only a
minimum stiffening of the femur will occur with this cap prosthesis.
Although the cap prosthesis described finds most use in human
implantation, it is also suitable for veterinary purposes.
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